Insurance law has grown out of the Law of contracts...in this case surety contracts. In the 1700's the English courts were merciless in the strict enforcement of contracts. Your question presupposes that the underwriters simply agreed to guarantee the shipment without verification of what they gauaranteed.

Records were required to have been kept by law. The sailors were generally paid upon their return from the trip. Very often they were required to supply their own clothing and tools of their trade. If they needed to buy new clothes or tools to replace broken ones they bought them from the ship's store on board the ship. Upon their return the expenses on their purchases were deducted from the payment they received. It was a matter of meticulous records having to be kept to protect the ship owner. Padding the account was not in his best interest unless he could foresee that the ship would be lost in a storm